Assiomqb of one-half to



Oct. 29, 1929. 5. J. PLIMPTON ,2

METHOD AND APPARATUS FOR TRANSLATING SOUND Filed NOV. 25, 1927 Inventor. Samuel J. Plimpton byXuMi ft s Patented Oct. 29, 1929 UNITED STATES PATENT OFFICE sAmuEL .1. rum-Peron, or woEcEsrEE, mAssAcEnsErrs, Assmnon or ONE-HALE T0 ROBERT BRUCE STURRUP, OF V70BSIES'IIEKR,v

MAssAcHusEMs, AND o'NE-fiALE iro EDWARD w. HANNA, 0E LEICESTER, mAssAczausEms marten AND APPAEArus re n mmsmrme soun'n Application fi1ed= Novenibe'r 25, 1927. sariai Nd. mm.

This invention relates to improvements in methods and apparatus for translating sound by impressing vibratory movement upon a region of an elastic vibratile structure having relatively greater area than said region, and the object of the invention is to reduce the resonance caused by the characteristic frequency of vibration of the elastic vibratile structure when subjected to vibratory move ment, and thereby eliminatedistortion of the sound transmitted thereby,

It is well known that when a vibratory motion is applied to a region of an elasticvb bratory structure, such as a disk, cone, or panel, the motion in general will be transmitted: to theboundaries and: there reflected back into the: region upon which such vibrations are impressed.- In all such cases there exists frequencies of vibration such that when the impressed vibration contains one-or more of thesefrequencies there are waves returningfrom the boundaries which are instep with eme advancing from the region" upon which. the vibrations are impressed in: sucha way as to produce so-calle'd: standing. waves or resonance vibrations.

Any such structure when used: in a; sound translating device, such. as a. loud! speal err,.-behaves: selectively with IG' REd' to these char.- acteristic frequencies with a c'onsequentdistortiron of the sound: or-mnsic which is being transmitted.

When a. wave is produced at the region to which the vibratory movement is applied, it travels to the boundary of the structure, that is to aregion where the elastic constant of the mediumchan-ges somewhat suddenly, and is there reflected. The reflected wave starts back toward the region: upon which: it is impressed. in' a: change of phase which-dc end s upon: the character of the-boundary;- i the boundary isabsolutely rigid the change of phase will be zero-and: the reflected wavewil'l returnin a: phase with the impremedi wa ve. Izf the boundany is perfectly free there will be; a: reversal or phase shift? ofthe'r'efl ected wave relatively to the impressed wave.

I have discoveredi that: thephase sh iffi" of portionso fizthe :vi'brart-ory waves reflecte'd from the bound'az'ies of an-el'asticvibrati-le' structure may be utilized: to: neutralize the effects of the reflected wa'vcsupon the region to which 'tliivibratory movementiisapplied so that the region to which the vibratory movement is applied will; bee nodal point with respect to the 'reflecwd waves thereby substantially eliminating: the effect Ofstanding waves and substantially preventing' the reaction of the refleeted-waves upon theregion to which vimevement is'applieti; I- have dis covered that this method is'applieable; to all wave-lengths whenever th arrangements at the boundaries: of the elastic vibrati le structures-re such thatie applies to" any one Wave e u reas nofthe present invention, therefisre, interference with the vibrations trans mitted to the Vibffilfillt? structure is uninipaired, refioriafiefe ftfiie 156- the chaiactr'istic metastases-vibrates of such Strtitiire' are sebstaaitralr el imfmaiea, am the sound transmieeee'bymeviBraeiufiemie-erasaidvibratile tructureeerresposasssesranfiaily perfectly to 'tli'sdtmd vibrati-eneimpressed uponit;-

' More pdfticularlythe"ptesent-nfethod earn;- prises translating sound by impressing. Vieras-n37 incveifient'fipon eregronefen elastic vibratil 'straeture cf relative? greater area than said region and restraining thevibrafiion oiipredeterintnetfre' s o the Bound eries'of satdlastic structfirern such a manner asto vary thing-Bases cf of the vibratory waves eeted from the restrained mgiens relesivei j to the phases-of the waves therebet-ween in sucha manner as substantieny te iieutralize the cheese ef-the reflected waves aporeehe region to which the: vibratory movement :eQppIi'ed-i- Desirahwthe restraint upon the predetermined" regiens of-the' Boundaries isdhrposed by a; movable or forces acting upon said regions of -the boundaries' im directions oppositei tb the displacement thereoficaused by the waves: imparted: to the stmct ure by said vibrafingtmove'gnenh. but it wilt also be understoodthat the? re's'trainfii may: beimposed uponi said regions: by a substantially immovable force.

The imamtioirv @186 comprises the? method of: translating: soundsbydmpressin'g vibratory movement upon the outer face of the central region of an elastic vibratile structure of relatively greaterarea than said region which presents a concave or re-entrant sound-pro- 'ecting face by restraining vibration of predeterrnined regions of the boundaries of, said elastic structure in such a manner as to vary the phases of portions of the vibratory waves reflected from ther'estrai ned regions relatively to the phases of "waves therebetween sufliciently to substantially neutralize the effects of the reflected waves upon the region to which the vibratory'movem'ent is'applie'd. In usual sound translatingdevices, such a s loud speakers, the vibrations are transmitted from the convex surface of a .coneor concavo convex disk the boundary of which is either free or is restrainedzthroughout..In either case standing sound wavesare developed which interfere; with "the; proper reproduction or ,translationiof the-.-music or other sound. Bythe method embodying the-present invention such undesirablewavesiareavoided and interferencewith the properimpresSion of vibratory movementuponthe elasticistructure is eliminatedlfFurthermore, byqreason of this invention the advantages ofthe more effective transmission of, the sound froma re-entrant face are obtained as the column of air which lies in front of the re-entrant face, whether of conical form or of concave .form, is confined to a-greaterorless degree .in accordance with the 90n1c1ty.or. concav ty of the surface, presented by-the i-Yibratory structure and sueh partial; confinement of the column of air increasesitheeffectiveness of the sound transmission, both as to strength and quality, very greatly. relativelyto that transmitted from the usual flat or convex surfaces employed in loud, speakers andvthe like. f n,

vFurthermore, reason .of. the present method-the resonance ofthe elasticvibratile structure is substantially eliminated and, therefore, the sounds transmitted are not subject to such resonance as isproduced by the usual horn type-of loudgspeakersin which .v1 bration is imparted to the column of air within the born from a relatively smallvibratile diaphragm Y By proper application of restramt to predetermined regions of the boundaries, which may be determined by experimentation or by .a somewhat complex mathematical calculation which need not be herein described, :the phase of different portions" of' the reflected waves may-be relatively varied or shifted in such a manner as to produce a nodal point of the reflected waves at the region upon which the vibrations are impressed, thereby substantially eliminating the effect of the characteri'stic sounding waves or resonance of the structure. w w

The method'above described-may be employed to translate sound from various types of elastic vibratile structure, such as cones, disks, or panels. Desirably a vibratile struc ture in the form of a cone is employed as it can be constructed of material having relatively small mass and greater rigidity than other types of elastic vibratile structures. The method herein described will, therefore,

"ts Hare fully described in its application to a conical sound translating device such as the conetype of'loud speaker employed in radio receiving apparatus.

An illustrative embodiment of the invention is shown in the accompanying drawings, in which,

Fig. 1 is a vertical central sectional View through a preferred type of conical loud speaker; V Y

Fig. 2 is a front elevation of the same on .a. somewhat reduced scale.

. The apparatus. illustrated comprises a casing -1,..which. may. beof rectangular or other form, to enclose the usual electrical mecha nism=by means ofwhich a rod or plunger 2 is vibrated which in turn imparts vibration to the elastic vibratile structure, such as a cone 3, made of suitable light material of an elastic, but substantially non-resilient character, suchas blotting paper or preferably a sheet product known commercially as .leatherette. The rod or plunger 2 desirably is connected to the apex of the cone by relatively rigid clamping members 4 and 5 ofcthin light metal such as aluminum.

:-.The casing 1 .may be supported in any suitable manner, as for example by screws extending. through-the back thereof into a wall, or. suspended by acord secured to a suitable eye 6. anchored in the casing, but .preferablyis supported by an easel construction comprising a rear leg 7 and a bracket 8 having legs 9 and'lO-to co-operate with the leg -7 and which bracket is secured to a rigid frame 11 which surrounds the boundaries of the cone. The frame illustrated herein is of regular polygonal form presenting twelve sides, but it will be understood that the number of sides of the polygon, or the form of the supporting frame may be varied as may be required by the number of points of restraint required, or that the frame may be of circular or other form complementary to the form of the vibratory sound translating structure employed.

The particular form of frame is illustrated herein as indicating a desirable structure which will present a maximum and uniform strength as anchorages for the restraining members 12 which desirably are secured at the unctions of the sides and extend radially therefrom to their points of attachment to separated regionsof the boundaries of the cone.

The restraining members 12 desirably are inextensible flexible cords which are suitably secured at' predetermined intervals to the boundaries of the cone and, as before stated, are anchored to the frame 11. These flexible restraining members extend substantially radially with respect to the axis of the cone and are adapted to permit the slight bodily vibration of the cone in the direction of its axis in response to the vibrations imparted to it through the rod or plunger 2, but the inextensible quality of the tension members offers a movable force acting in opposition to the vibration of the boundaries of the cone caused by the vibratory waves radiating from the center of the cone and reflected back from the edges thereof so that the regions of restraint of the boundaries of the cone are in effect substantially rigid relatively to the regions intermediate of the restrained regions so that by properly spacing the restrained regions of the boundaries the phase shift of the reflected waves returning from the restrained and relatively free regions of the boundaries will have a nodal point at the apex of the cone, thereby neutralizing the effect of he characteristic standing Waves of the co e upon the mechanism through which vibration is imparted to the cone. Furthermore, the substantial neutralization of the standing waves permits the vibration of the cone in true consonance with the vibrations imparted to it through the actuating rod or plunger 2 and consequently translates such vibrations into sounds which are substantially pure and unaffected by such undesirable vibrations as are inherently produced in vibrating cones, or other structures, in which the edge is either wholly free or Wholly restrained or improperly partially restrained.

It may be noted that the elastic inextensible restraining members 12, which are illustrated at the upper and lower boundaries of the cone 3 in Fig. 1, appear to extend in an angular direction relatively to the plane of the boundary of the cone. Such illustration,

of course, is greatly exaggerated and the members are shown in this position merely to illustrate the suspension of the cone which is mainly supported from its top with its center of gravity to the left of the plane of the boundary of the cone which naturally causes the lower portion of the cone to swing to the right and the upper portion to the left. The action, however, of these restraining cords upon the boundaries of the cone is substantially radial of the axis of the cone and the movement of the boundary of the cone caused by the impressed vibratory waves and the reflected waves is resisted by the resultant of the tension imposed upon such restraining member which acts in a direction opposite to the direction of movement imparted to the boundary by such waves.

lVhen vibratory movement is imparted to the cone at its apex by the vibratory mechanism of a radio receiving set, or by other vihratoraz means,- Ithe vibrations are imparted to the apex-region. iof. rthe cone and the cone vibrating incousenanoe therewithtransmits e. vihration directly to the colamuof air in front Orf the coneave iaceof the cone. The F vibratory- ;waves which :mdiate fnomthe apex region of the-acne to the boundar s 't mo are (reflected *from the beuhdarieshut by reas'onof the restramtofpredetermined regions of the boundaries the phase shift of falternatingjgwrt-iqns (Qt the reflected waves pro- .duces anodal potataf the reflected Waves'at the-apexof the ,cone, and neutralizes the effect of standing waves ;,upon.;,the vibration of the {cone and eliminates iaterifeaenee of ,the stand- :ingwawes withtheaetioaof the mechemism which imparts wibratory movement to the ap e s h -sans a ,v t l have d monat at by pmt ica w m .tion aha at mew t-tr mit ag 809. 1 as herein descqihed isjgvastlysqpenior to ether methods beeauseqofi-the elimination .ofistanding aves aafithel vaid m at in e s *by undesirable vibrations with .aa i'vibratouy movement which islimparted to the sound transmitting-structure. g

mere awlgelatiyetyrlaggeeone is employed t ns h efi vaadie e n o wax mately Mouth of thelonges't wave emgth ,to be transmitted, ,a,-substantially erfeetreotmusie transmitted there.- om, I

i -;It. willbe. aaer aaaaa ,a m aea illustrated and describedfherein for the. periformanoe -01 'the,- odofgtransmitting herein described is of .an illustrative .eharaeterandis aotmestrictive and thatsneh ether tape f ppem s ii ae he h io me tien d, may h em y i -thegpeiacr ,ea within the spirit and scope of the following claims,

' Having thus deserihed-theinvention, what fishlaimedas 411ew;, .and;desired itohe. secured ,lgyLetters-Batent, a .1; The method of translating 011118. m aa-wi sa r eme e e up a -negi n of;-anelastic vi ,ratilestructureotrela- .tiaebn'g neaten ateathan said region andcaus- .ing .aphaseshitt-oi ,portians of the vibratory waves/reflected ,tromthe boundaries of said 5 structure to. meutralizetheefieets :olf reflected awn/fies upon. the region to -whic h said yibraitorymovemenhisa .v l

2- llhelr'nethod-ofitranslatingsoundby 'impressing vibratorymovement u on aregiion -afiantelastie stnuetareofrelati-vefy, reatarea, and causing alterna-tin portionso waves re- ;fiectedirom-thehoun ies ofsaidstructure to vary in phase insuch mannerasspbStantialrliy tomeutralizze the'efiects of the reflected waves upon the region to which said vibratory movement is applied.

3. The method of translating sound by impressing vibratory movement upon a region of an elastic structure of relatively great area,

and restraining the vibration of predetermined regions of the boundaries of said elastic structure in such manner as to vary the phases of portions of the vibratory waves reflected from the restrained regions relatively to the phases of said waves therebetween sufliciently to substantially neutralize the effects of the reflected waves upon the region to which said vibratory movement is applied.

4. The method of translating sound by impressin vibratory movement upon a region of an ei astic vibratile structure of relatively greater area than said region and effectively restraining the vibrations of predetermined regions of the boundaries relatively to those of intermediate regions thereof by movable forces acting upon said'boundaries in directions opposlteto the displacement thereof caused by the'waves'imparted to said structure'by said vibratory movement.

5 The method of translating sound by impressing vibratory movement upon the convex central region pf an elastic structure of relatively greater area than said region and which presents a re-entrant sound-projecting face and restraining the vibration of predetermined regions of said elastic structure in such a manner as to vary the phases of portions'of the vibratory waves reflected from the restrained regions relatively to the phase of the waves therebetween sufliciently to substantially neutralize the effects of the reflected waves upon the central region to which the vibratory movement is applied and thereby enable the elastic structure to impart vibrations to the partially confined column of air in'contact with the re-entrant face thereof unaffected by standing waves or resonance characteristic of the vibratile structure. i

6. A device for translating sound comprising a vibratile conical structure having a diameter approximating one-fourth of the length 'of the maximum wave to be transmitted, means for imparting vibratory movement to a relatively small region of the apex of said cone, a rigid support for the boundaries of said cone and means anchoring regions of said boundaries to said support at predetermined intervals to provide relatively free portions of said boundaries therebetween operable to cause a phase shift of portions of the waves reflected from said boundaries of such character as to cause the system of reflected waves to have a nodal point at the apex of said'cone.

In testimony whereof, I have signed my name to this specification.

A SAMUEL J. PLIMPTON. 

